The present invention relates to an echo sounder of the type composed of a transmitting arrangement including two transmitting transducers for the simultaneous pulse-shaped transmission into a body of water of sonic energy at two high frequencies which differ slightly from one another. The intersection of the radiation of this sonic energy at the two different frequencies generates energy which radiates in the water and which has a low frequency equal to the difference between the frequencies emitted by the two transducers. Such echo sounder further employs a receiving arrangement including a low frequency receiving transducer for receiving the echo signals at the difference frequency and a display device for recording the echo signals according to their propagation times, the display device having a signal input which is connected with the low frequency receiving transducer.
It is known to use echo sounders operating with a plurality of transmitting frequencies for the differentiation of bottom strata in a body of water. A safe distinction between hard and soft bottom strata in a body of water is a necessity for marine vessels, particularly in areas where such vessels travel with very little clearance under the keel, and in muddy or silty waters possibly even in contact with the bottom. Such a situation is commonly encountered by supertankers, for example. In such areas it is necessary to continuously monitor the bottom for hidden objects in a layer of mud which could endanger the vessel.
German Pat. No. 1,017,054, for example, describes an arrangement which by transmitting sonic energy pulses at two transmitting frequencies permits a differentiation of bottom strata. Sonic energy at a high transmitting frequency is reflected at the upper stratum of the bottom and does not penetrate therein, thereby furnishing echo signals which indicate the location, or depth, of this upper stratum.
Sonic energy at lower transmitting frequencies penetrates into the bottom and thus furnishes echo signals from the lower lying deposits at the bottom.
However, difficulties arise with this mode of operation with respect to the display of the received echo signals on a common display device. These difficulties have their origin in the fact that with echo sounders the direction characteristic of the sound energy is very narrow for high transmitting frequencies and very broad for low transmitting frequencies. A broad directional characteristic leads to the known effect of a seeming extension of the bottom echo since the reflected sound energy comes not only from parts disposed vertically underneath the echo sounder but also from adjacent parts of the bottom, the latter signals arriving at the receiving transducer staggered in time with respect to the echo signal from the vertical direction. Such an extended bottom echo may mask echo signals from lower lying strata appearing during the same time duration of the bottom echo.
A further difficulty in the display of received echo signals at different frequencies on a common display device arises in the display of echo signals from sloped areas in which the echo signals at the lower frequency may, under certain circumstances, arrive earlier from the slope than from the bottom, whereas the echo signals at the higher frequency always permit a display which reflects the proper bottom orientation. This leads to overlaps in the display, as they are described, for example, in Volume 1 of "Beitrage zur Schallortung" [Contributions to Echo Sounding] on page 51 and illustrated in FIG. 19 (following page 51). This volume was published as part of the series "Sonderbucherreihe der Funkortung" [Series of Special Books on Radio Ranging] by the Committe on Radio Ranging and contains lectures given by the working group on echo sounding at the Symposium in Bremen on Oct. 19th, 1953. Publisher is State Secretary Professor L. Brandt.